1. Field of the Invention
The present invention relates to an optical scanning system used for a laser printer or a similar optical device. More precisely, the invention relates to an optical scanner having a scanning lens with a correcting lens which compensates for aberration.
2. Description of Related Art
In the case that color printing is to be done, or if it is necessary to increase the resolution of an object to be printed, the diameter of the optical scanner beam must be as small as possible to thereby enhance the resolving power. For a small and compact printer to print on a large printing medium, it is necessary to increase the scanning width (i.e., range) as much as possible.
As a solution to the above problems, it is known to provide an aberration correcting lens in a beam scanner, in addition to a conventional f.theta. lens which functions to form an image. Furthermore, in a known optical scanner having a polygonal mirror as a deflector, the bundle of beams is usually converged in the sub scanning direction in the vicinity of the polygonal mirror to prevent the scanning line from being deviated in the sub scanning direction due to inclination of the polygonal mirror surface (from an upright position). Consequently, the power is increased in the sub scanning direction and, accordingly, the curvature of the field, etc., becomes extreme, particularly in the sub scanning direction.
The optical system having the aberration correcting lens in addition to the f.theta. lens is disclosed, for example, in Japanese Unexamined Patent Publication No. SHO61-120112. In this optical system, there is a deformed cylindrical lens (i.e., toric lens) having curvature within the sub scanning surface which decreases from the optical axis towards the periphery between the two f.theta. lenses which mainly function to form an image and an image surface. With this arrangement, since the position of the image to be formed within the sub scanning section of the image surface is moved towards the polygonal mirror in the central portion of the image surface, and away from the polygonal mirror in the peripheral portion of the image surface, the curvature of the field within the sub scanning section can be reduced.
The toric lens, disclosed in JPP '112 mentioned above, has a curved surface defined by a locus that is obtained by revolving an arc having a radius of curvature Ry in the main scanning direction about an axis y as shown in FIG. 13. The radius of curvature Rz in the sub scanning direction is determined in accordance with the radius of curvature Ry in the main scanning direction. Accordingly, if the image forming position within the sub scanning section irregularly changes, depending on the efficiency of the image forming lens, etc., or due to the change in the position of the deflection point of the polygonal mirror, it is impossible to completely correct the curvature of the field.
The correction can be carried out by using a deformed toric surface in which the radius of curvature in the sub scanning direction is determined independently of the radius of curvature in the main scanning direction, as disclosed, for example, in Japanese Unexamined Patent Publication No. HEI2-39120 or HEI2-46418.
In the scanning optical system disclosed in JPP '120 mentioned above, the deformed toric surface is located between the polygonal mirror and the f.theta. lens at a position in which the magnification is high, i.e., at a position away from the image surface. In general, the deformed toric lens is made of molded plastic since its shape is complex. However, the refractive index of a plastic lens largely varies depending on the change in the temperature. Moreover, plastic lenses have a high rate of linear expansion. Consequently, the location of the plastic toric lens away from the image surface causes the focal point to change considerably due to the temperature variation.
In the scanning optical system, disclosed in JPP '418 mentioned above, the deformed toric surface is formed on the surface of the f.theta. lens. In this arrangement, if the f.theta. lens, provided with the deformed toric surface, is made of plastic, since the power thereof is large, there would be a considerable displacement of the focal point due to the temperature change. If the f.theta. lens, provided with the deformed toric surface, is made of glass, a precise grinding machining process is necessary. Accordingly, the f.theta. lenses can be neither mass-produced nor inexpensively manufactured.